Reflecting a tendency to prepare larger diameter silicon wafers and fabricate higher precision devices therewith, requirements for finish precision (thickness uniformity, flatness and smoothness) of a silicon wafer subjected to polishing finish (polished wafer) have been increasingly enhanced.
In order to satisfy such requirements, efforts have been made to attain a higher level in wafer polishing technique, and development and improvement of polishing apparatuses have been carried out.
As one example thereof, so-called single wafer polishing apparatuses have been newly developed for the purpose of polishing a large diameter wafer, especially 300 mm or more in diameter, and some of them have been practically used.
In the single wafer polishing method, however, there arise problems: for example, (1) requirements for reduction in wafer cost is hard to meet in terms of productivity, and (2) recent requirements for wafer flatness as far as an peripheral area adjacent to the wafer edge (within 2 mm) cannot be sufficiently satisfied.
Meanwhile, there has been widely used a batch type polishing apparatus in which a plurality of wafers are simultaneously polished. An outline of a configuration of a portion of the apparatus directly associated with polishing action is shown in FIG. 19. In this polishing apparatus, one or more wafers W are held by means of such as adhesion on a lower surface of a work holding plate 13 rotated by a rotary shaft 18; to-be-polished surfaces of the wafers W are pushed, for example, using a top weight 15 onto a surface of a polishing cloth 16 adhered on an upper surface of a polishing table 10, which is rotated at a prescribed rotational speed by a rotary shaft 17; and a polishing agent solution (hereinafter may be referred to “slurry”) 19 is simultaneously supplied at a prescribed rate onto the polishing cloth 16 through a polishing agent supply pipe 14 from a polishing agent supply device (not shown). In such a situation, polishing of the wafers W are performed while the to-be-polished surfaces of the wafers W are rubbed by the surface of the polishing cloth 16 in the presence of the polishing agent solution 19 therebetween.
In this batch type polishing apparatus, there is increasing difficulty in satisfying requirements for precision of finish surfaces of the wafers in a trend of transition to larger-sized apparatuses in company with larger diameter wafers for the following reasons: deflection of a polishing table and work holding plates by weights thereof and polishing pressure, and thermal deformation by heat generation in polishing action; and in addition thereto, deformation and displacement of the polishing table and the work holding plates caused by various kinds of mechanical deflections in rotation thereof.
In order to cope with such problems, various kinds of ingenious contrivances have been practiced about a structure and materials, and operating conditions of the polishing apparatus and other polishing conditions. For example, some of contrivances on the structure are as follows: (a) in order to prevent thermal deformation of a polishing table, as shown in FIG. 20, a separate lower table 23 on which multiple recesses 21 for circulating a cooling water H are formed is provided on a lower surface of an upper table 12 on an upper surface of which the polishing cloth 16 is adhered; further, ribs are provided on a lower surface of a polishing table to prevent deformation due to polishing pressure; and still further in order to effectively suppress thermal deformation, contrivances have been piled up about a structure of a polishing table and arrangement of flow paths of cooling water, as shown in JP-A-95-52034 and JP-A-98-296619.
In a prior art polishing table shown in FIG. 20, however, there is adopted a structure in which an upper table 12 made of SUS410 and a lower table 23 made of cast iron such as FC-30 provided with flow paths for cooling water are coupled to each other by fastening them with clamping members 11 or the like, and a temperature difference between the upper and lower surfaces of the upper table arising in the course of a prior art polishing operation is generally 3° C. or higher and, in higher cases, 5° C. or higher; therefore, a difference in height (deformation) at a highest or lowest point occurs inconveniently in places on the upper surface of the upper table amounting to 100 μm or more relative to the reference plane, namely the upper surface of the upper table with no temperature difference between the upper and lower surfaces thereof.
Furthermore, the following proposals have been made: (b) that a material with a low thermal expansion coefficient (8×10−6/° C.) is used as a material of a polishing table (WO94/13847), that a polishing table is of a one-piece structure made of ceramics in which a flow path for circulating cooling water is formed throughout almost all of the interior (JUM-A-84-151655), and the like techniques; and in addition, (c) that a temperature control fluid is likewise circulated in a work holding plate for the purpose of improving temperature uniformity across a wafer holding surface of the wafer holding plate (JP-A-97-29591).
Moreover, in order to suppress a temperature rise of a wafer and a polishing cloth due to heat generation accompanying polishing action, the following procedures have been performed: in addition to the cooling of the work holding plate and the polishing table described above, a cooling function is also given to a polishing agent solution (in usual case, a weak alkaline aqueous solution mixed with colloidal silica is used.) supplied directly onto a polishing action surface, an amount of the polishing agent solution exceeding a supply amount necessary for polishing action in a pure sense is supplied onto the polishing cloth, and the polishing agent solution discharged from a polishing site is recycled in order to reduce the cost.
In the construction of the prior art polishing apparatus and a cooling method as described above, a temperature on a polishing cloth surface during polishing gradually rises from the start of polishing and a value of the temperature at a portion where the polishing cloth is put in contact with a to-be-polished surface of the wafer rises usually to 10° C. or higher and a temperature at a corresponding upper surface portion of the polishing plate direct under the portion of the polishing cloth in the contact also rises by 3° C. or more.
On the other hand, changes in temperature on the lower surface of the polishing plate are restricted to 1° C. or less by virtue of an effect of suppression of a temperature rise by cooling water. Therefore, a temperature difference of at least 3° C. or more arises not only between the upper and lower surfaces of the polishing table, but also between a high temperature portion and a low temperature portion on the upper surface of the polishing table, which causes a portion of the upper surface of the polishing table with thermal deformation/displacement of 100 μm or more in a direction normal to the upper surface of the polishing table in comparison with that when no temperature difference exists.
Furthermore, a work holding plate has become larger in size in response to transition in diameter of a silicon wafer toward a larger value. For example, in case of a work holding plate for use in polishing of 8 inch wafers, a diameter of the work holding plate assumes about 600 mm and a weight thereof also increases as the diameter increases.
Accordingly, not only thermal deformation of a work holding plate caused by heat generation at a polishing surface but also deformation caused by a weight of the work holding plate are problematic; therefore, various trials have been performed in order to suppress such deformation: to increase in thickness of a work holding plate or to decrease deformation by use of a material whose modulus of longitudinal elasticity is large, such as ceramics (silicon carbide and alumina).
Moreover, in a prior art batch polishing, as shown in FIG. 21, for example, a method was adopted in which a to-be-polished wafer W is adhered on a work adhesion surface 20a of the work holding plate 20 with an adhesive 22 applied therebetween.
In this case, it is important that no air bubble is left behind in a adhesive 22 layer and at interfaces between the wafer or the work holding plate 20 and the adhesive 22. For this purpose, a adhering process goes in the following way: as shown in FIG. 21, an air bag 27 expanding so as to be convex downward and provided on a lower surface of a pressure head 25 is pushed onto an upper surface (a surface opposed to the to-be-adhered surface) of the wafer W by the action of a pressure cylinder 26 and a contact surface under pressure of the air bag with the upper surface is increased by the push from the central portion of the to-be-adhered surface of the wafer sequentially part by part toward the periphery thereof such that air in the adhesive and at the adhesion interfaces are driven out and beyond the outer edge of the periphery of the wafer. However, while air in a boundary layer between an adhesive and each of the wafer W and the work holding plate is expelled by such a push-out method with a wafer pressure member 24, a thickness of the adhesive layer 22, on the other hand, is apt to be thinner at a central portion of the wafer W, which causes an inconvenience that the wafer W is fixed in a distorted state.
While, in the prior art, natural rosin, synthetic rosin ester, beeswax, phenol resin and so on were employed as adhesives for use in adhesion of a wafer taking into consideration various factors such as dissolution resistance to a polishing agent solution, a non-lubricating property, a change in characteristics due to a temperature rise of the adhesive through a temperature rise due to polishing heat generation, adhering action by such adhesives is mainly dependent on a physical adhesion mechanism, which goes like this: After an adhesive dissolved in a solvent is applied on an adhesion surface of the wafer holding plate, the solvent is evaporated off, and then, a wafer is pushed onto the work holding plate at a prescribed pressure while keeping the adhesive in a softened state under heat application and thereafter, the adhesive is solidified by cooling to a normal temperature to complete the adhesion.
In such an adhering process, it is necessary to heat a wafer and a work holding plate at a temperature, for example, ranging from 50 to 100° C. and improvement on processing precision is retarded by deformation of the wafer and the work holding plate caused by a thermal history in the heat treatment. In addition, there are required special apparatuses and facilities, and energy consumption for such heat treatment and others, which has also become problematic in an aspect of cost.
On the other hand, so-called normal temperature adhesives that have been available, which exert adhering action at normal temperature have not been able to be used in a practical aspect because of weak points such as low dissolution resistance to a polishing agent solution, difficulty in separating a wafer from a work holding plate and difficulty in removing the adhesive from a work holding plate.
Furthermore, in order to prevent air bubbles from being left behind in an adhesive at an adhesion site, the following processes have been practiced: a method in which a to-be-adhered surface of a wafer is pressed onto the work holding plate with an adhesive therebetween while holding the to-be-adhered surface of the wafer so as to be inclined to a work holding surface of the work holding plate and a contact surface is increased by the push from the one edge of the wafer sequentially part by part toward the edge opposite to the one edge thereof such that air in the adhesive between the to-be-adhered surface of the wafer and the work holding surface is expelled from one edge of the to-be-adhered surface of the wafer toward the edge opposite to the one edge thereof, a method in which as shown in FIG. 21, an elastic member having a convex front shape(air bag) 27 is pressed onto the upper surface of the wafer W placed on the work holding plate 20 while increasing a contact area part by part sequentially from the central portion of the wafer toward the periphery of the wafer to expel the air to the outside; and a method in which the whole of the work holding plate 20 or each wafer W is sealed by a holding surface of the work holding plate 20 so as to be air tight and the interior space closed by the sealing is evacuated into a reduced pressure state, whereby no air is left behind.
In FIG. 22, 1 indicates a vacuum vessel; 2, bellows; 3, a cylinder for vertically shifting bellows; 4, an internal pressure adjusting pipe for bellows; 5, an internal pressure adjusting pipe for a vacuum vessel; 6, a vacuum suction pipe; 20, a work holding plate; and W, a wafer.
A fault that a thickness of an adhesive layer becomes non-uniform (equal to or more than 0.5 μm) is problematic in a method shown in FIG. 21 in which a to-be-adhered surface of a wafer is pushed to increase a contact area sequentially part by part from a portion of the to-be-adhered surface thereof, while problems arise such as that a special apparatus and special tools are required and a process is complex, and in addition that dust is generated from the apparatus and tools in a method shown in FIG. 22 in which a wafer or all of a work holding plate is placed in a vacuum state to complete adhesion.